The present invention relates to a method and apparatus for motion monitoring and is particularly described in an embodiment for detecting episodes of apnea in infants.
Research into the problem of infant crib deaths or sudden infant death syndrome (SIDS) in which infants have died during sleep have generally indicated that a significant percentage of infants up to the age of about one year may have problems in the central nervous system reflex such that, during sleep, respiration may cease, a condition referred to as apnea. In most instances, the infant will begin breathing again spontaneously. However, if the duration of an apnea episode is excessive, irreversible cerebral damage may be sustained and, in some cases, death occurs. Clinical procedures have been devised for determining which infants are susceptible to onsets of apnea but are relatively inaccurate.
When an infant is diagnosed as being susceptible to an apnea episode, it is necessary that some method be provided for monitoring the cardio-pulmonary activity of the infant during sleep so that the onset of an episode may be detected quickly and in such a manner that assistance may be provided to the infant to restart breathing. Various types of monitoring devices have been developed for detecting apnea episodes. Notable among these is that disclosed in U.S. Pat. No. 4,474,185 issued Oct. 2, 1984 to the inventor of the present invention. That patent discloses a motion monitor in which very small variations in dielectric constant are measured and used with appropriate timing circuits to provide an alarm when cessation of motion is noted for a predetermined period. Such motion is generally caused by breathing of the patient being monitored, such breathing being associated with motion of the chest and diaphragm. The monitor disclosed therein is essentially a pad which may be placed over a crib mattress or the like and upon which the infant or other patient may lie. In its basic form, the pad includes a plastic film having a zig zag like pattern formed on one surface thereof from a thin metallic foil thereby forming an electrode. An electrostatic shield is formed from a solid sheet of metallic foil which may be bonded to another surface of the first plastic sheet or may be bonded to a second plastic sheet which is then overlayed over the zig zag foil pattern of the first sheet. In either case, the resultant monitoring device appears as a pair of spaced conductive layers separated by an insulating layer.
In order to detect the change in dielectric constant caused by the motion of an infant or other patient adjacent the electrode, the electrostatic shield is connected to an output of a voltage follower, the input of which is connected to the electrode. An oscillator producing a 50 Khz output wave form is provided, and is earth grounded, such that the oscillator pumps earth ground with respect to an internal or floating circuit common such that any moving object between the active electrode and earth ground will act as a modulating dielectric and will change the amplitude of the 50 Khz carrier signal applied. A disadvantage of this system is that the monitor pad must be isolated from earth ground and the system requires a 50 Khz oscillator to generate the carrier signal by pumping earth ground.
A related type of arrangement is disclosed in U.S. Pat. No. 4,438,771 issued Mar. 27, 1984. The device of the latter patent includes a passive charge variation conductive pad spaced from the body which is being monitored and in which a potential is induced by movement of the body, such as, e.g., from the respiratory function or the cardiac function. The induced potential is amplified in order to produce a voltage output representative of movement of the body or change in capacitance or charge distribution on the conductive pad. It is proposed in this patent that movement of a body results in a movement of charge on the body which movement of charge induces a potential in the conductive material of the pad. It is also proposed that each human body or animal body contains a net or at least an accumulation of charge and that motion of the body results in movement of the charge. Although this latter patent uses the same or a very similar type of capacitance probe approach to many of the prior art patents, as distinguished from the single electrode monitor of applicant's U.S. Pat. No. 4,474,185, the system proposed in U.S. Pat. No. 4,438,771 has been found not to be a practical system. In particular, tests have shown that the system of U.S. Pat. No. 4,438,771 is sensitive to electromagnetic interference and may consequently fail to alarm for all apnea episodes. Such interference may comprise, for example, AM and FM radio signals, 60 cycle power line interference and many other high frequency signals generated by, for example, ignition systems of automobiles.